Science, and the rationalist movement in general, face a "sinister challenge" from leftwing thinkers who promote cultural relativism, according to evolutionary biologist Richard Dawkins.

He told a packed Hay Festival audience that although the threat from creationists and the religious right is well-documented, science is also under threat from the other end of the political spectrum: "I think we face an equal but much more sinister challenge from the left, in the shape of cultural relativism - the view that scientific truth is only one kind of truth and it is not to be especially privileged."

As an example, he cited Kennewick Man, the 9,000-year-old set of human remains found on the banks of the Columbia river in Washington State in 1996. The view of local native Americans that Kennewick Man was their ancestor, despite strong scientific evidence to the contrary, initially held sway, and they were able to put a stop to research.

Fellow panelist for the "Guardian Science Experiment" was the geneticist, Steve Jones, of University College London. Speaking to the event's tag line "Have we abandonned the enlightenment?", he reserved special venom for the animal rights movement, which he attacked as "stridently anti-rational".

He particularly objected to the claim that research on animals does not work. "The standard claim made again and again is that research on animals has killed millions of people ... scientists are fooling themselves in suggesting that it works," said Prof Jones.

The truth is the opposite, he said. Millions of diabetics, for example, are alive today because of the availability of insulin. The hormone was discovered through research on dogs.

"That's what I find most uncomfortable - the denial of scientific truth because of pre-formed beliefs," said Prof Jones. "That is what the enlightenment was set up to prevent and that is what I strongly feel has come back."

Earlier in the debate, Prof Dawkins had revealed that last year he received a Christmas card from the archbishop of Westminster - although not one from President Bush.

The third panelist, the President of the Royal Society, Sir Martin Rees, felt that he should grasp this olive branch with both hands because scientists needed to form an alliance with moderate faith groups in order jointly to fight fundamentalist religion.

"He should send Christmas cards to a few more archbishops," said Prof Rees, "on the grounds that if we give the impression that science is hostile to even the kind of mainstream religion that we have in this country, I think it will be more difficult for us to combat the kinds of anti-science sentiment that are really important."

But with Prof Dawkins now seemingly set on training his formidable intellectual artillery on politically-correct lefty thinking, the chances that he will expand his Christmas card list to cuddly archbishops seem pretty remote.

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